When black holes align: A subpopulation of aligned massive binary black holes observed via gravitational waves

In this work, we investigate the features present in the joint primary mass and effective spin distribution of binary black holes, without relying on specific modelling assumptions. We made use of non-parametric methods, flexible models capable of approximating arbitrary probability densities with minimal mathematical assumptions, applying them to the newly released GWTC-5.0. Our analysis supports, albeit with large uncertainties, the presence of at least two separate sub-populations of binary black holes displaying different effective spin distributions: one of them with a large primary mass (m_1≳50 M_⊙) and a preference for positive ̧hieff values. These results point towards systems formed in a non-spherically symmetric, dynamical environment.